The reaction conditions under which esterification is effected can be varied considerably. The reaction proceeds very slowly at room temperature, but quite rapidly at elevated temperatures. About 99% of the limiting reagent, e.g., acids, anhydrides or polyols, is converted to an ester within a few hours. Limiting reagents are typically reagents which are not present in stoichiometric excess, e.g., limiting reagents used to make plasticizers include diacids and phthalic anhydride and those used to make polyol esters are polyols.
In the production of esters by the reaction of an acid or anhydride with at least one alcohol or polyol, water is a by-product of the reaction. Since the reaction is an equilibrium reaction, it is forced to completion by the removal of the water by-product, typically through distillation of the water from the reaction mixture during the esterification process. Frequently, an entrainer is used to aid in the distillation of the water from the reaction mixture. Inert materials such as benzene, toluene, or xylene may be used as the entrainer. In addition, the reactant having the lower boiling point has also been employed as the entrainer. In this latter case, the reactant used as the entrainer is charged into the reaction mixture in excess over the stoichiometric quantities required for the reaction.
The conventional procedure is to charge all of the reactants into the reactor at the beginning of the reaction cycle. The reaction mixture is then heated and reaction begins. The temperature of the reaction mixture rises until the boiling point of the reaction mixture is achieved, at which point the entrainer and water by-product boil out of the reaction mixture. Typically, the overhead vapors are condensed, the water separated from the entrainer, and the entrainer recycled to the reactor vessel. The reaction temperature, and therefore the rate of reaction, are thus determined by the boiling point of the reaction mixture.
When the reactant with the lower boiling point is also used as the entrainer, its concentration is gradually reduced as the reaction proceeds. Thus the reaction temperature, and therefore the rate constant for the reaction, increases as the reaction proceeds.
The present invention is not only economically desirable, but it also substantially increases the overall rate at which the reactants are converted to esters. In general, for a given conversion the lower the water content, the faster the rate of reaction.
The present invention provides a novel method for increasing the rate of esterification reaction, wherein only a portion of the lower boiling point reactant is added to the reactor at the outset, followed by the staged or subsequent addition of the remaining portion of the lower boiling point reactant throughout the remainder of the esterification reaction. Since the concentration of the lower boiling point reactant in the reaction mixture is less in the present invention than in the conventional esterification process wherein all of the lower boiling point reactant is added to the reaction mixture at the outset, the temperature of the reaction mixture will be higher over time than the temperature in the conventional case. Consequently, the reaction temperature, and therefore the rate of reaction, will be higher during staged addition of the lower boiling point reactant than during conventional processing. The higher rate of reaction translates into a shorter reaction time for the staged addition process than for the conventional batch process. As the reaction proceeds and the concentration of the lower boiling point reactant is depleted, additional amounts of this reactant are added to the reaction mixture in stages to ensure that it is present in sufficient quantities to satisfy the reaction requirements. At the completion of the reaction, the same total amount of the lower boiling point reactant has been used in the staged addition process as in the conventional batch process wherein all of the reactants are charged to the reactor at the outset.
The present invention also provides many additional advantages which shall become apparent as described below.